1. Field of the Invention
The invention relates to an agitator mill for treating free-flowing grinding stock, comprising a grinding receptacle which defines a substantially closed grinding chamber by means of an inner wall; and an agitator which is rotarily drivably disposed therein and which is cup-shaped in relation to a common central longitudinal axis, having an annular cylindrical rotor which has a closed wall; and an interior stator which is disposed within the rotor and fixedly joined to the grinding receptacle; wherein an annular cylindrical exterior grinding chamber in the form of an annular gap is formed between the inner wall of the grinding receptacle and an outer wall of the rotor, the exterior grinding chamber having a radial gap width; and an annular cylindrical interior grinding chamber in the form of an annular gap is formed between an inner wall of the rotor and an outer casing of the interior stator, the interior grinding chamber being arranged coaxially within the exterior grinding chamber and connected thereto via a deflection chamber and having a radial gap width h; wherein the exterior grinding chamber, the deflection chamber and the interior grinding chamber constitute the grinding chamber which is partially filled with auxiliary grinding bodies; wherein a grinding-stock supply area, which is disposed upstream of the exterior grinding chamber and opens into it in the direction of flow of the grinding stock, and a separator device, which is disposed downstream of the interior grinding chamber in the direction of flow, are disposed approximately on the same side of the grinding receptacle for the grinding stock to pass through; wherein auxiliary-grinding-body return conduits are provided in the agitator for returning the auxiliary grinding bodies from the vicinity of the separator device into the grinding-stock supply area, the return conduits connecting the end of the interior grinding chamber to the beginning of the exterior grinding chamber; and wherein the inner wall of the grinding receptacle and the outer wall and the inner wall of the rotor are free of interruptions, and the inner wall of the grinding receptacle and the outer wall of the rotor are smooth and free of agitator implements.
2. Background Art
In an agitator mill of the generic type known from U.S. Pat. No. 5,950,943 the interior grinding chamber as well as the exterior grinding chamber are smooth-walled without any interruptions and free from agitator elements. The gap width i.e., the radial extension of the exterior grinding chamber, distinctly exceeds that of the interior grinding chamber. This is meant to accomplish that grinding and dispersing the free-flowing, slurried grinding stock takes place predominantly by shearing effects in such a way that the local intensity of strain on the grinding stock is substantially constant throughout the entire grinding length of path. The smooth-walled design of the cylindrical boundary walls of the exterior grinding chamber and the interior grinding chamber produces a flow in which the auxiliary grinding bodies are moved relative to each other in layers. The shearing gradient and thus the local intensity of strain is constant over the respective grinding-chamber height in the exterior grinding chamber on the one hand and in the interior grinding chamber on the other. With the gap width of the interior grinding chamber being smaller than the gap width of the exterior grinding chamber, the shearing gradient can be made equal in the exterior grinding chamber and in the interior grinding chamber; it is then virtually constant throughout the grinding chamber. Problems have turned out to be posed by the fact that start-up of the agitator mill is difficult in the case of a high auxiliary-grinding-body fill factor. Because of its start-up problems the agitator mill is operated at a reduced auxiliary-grinding-body fill, which again leads to unfavorably rough distribution in grinding-stock particle size. Since this reduction of auxiliary grinding bodies reduces the amount of auxiliary grinding bodies that return through the auxiliary-grinding-body return conduits, there is an increase in the risk of so-called grinding-stock shooting flow i.e., grinding stock that has been supplied to the grinding-stock supply chamber for being ground or dispersed may short-circuit through the auxiliary-grinding-body return conduits towards the separator device.